This invention relates in general to thermal imaging compositions, and to direct-write imaging members (particularly lithographic printing plates) prepared therefrom. The invention also relates to a method of imaging such imaging members, and to a method of printing using them, with or without processing after imaging.
The art of lithographic printing is based upon the immiscibility of oil and water, wherein an oily material or ink is preferentially retained by an imaged area and the water or fountain solution is preferentially retained by the non-imaged areas. When a suitably prepared surface is moistened with water and ink is applied, the background or non-imaged areas retain the water and repel the ink while the imaged areas accept the ink and repel the water. The ink is then transferred to the surface of a suitable substrate, such as cloth, paper or metal, thereby reproducing the image.
Very common lithographic printing plates include a metal or polymer support having thereon an imaging layer sensitive to visible or UV light. Both positive- and negative-working printing plates can be prepared in this fashion. Upon exposure to a patterned light image, and perhaps post-exposure heating, either imaged or non-imaged areas are removed using wet processing chemistries.
xe2x80x9cDirect-writexe2x80x9d imaging avoids the need for patterned light imaging and chemical processing. Direct-write using an infrared radiation laser is a thermally driven process and is more desirable because the laser heats only a small region at a time. Moreover, computer control allows for high resolution images to be generated at high speed since the images can be produced directly on the imaging member surface, pixel by pixel. The chemical processing steps are eliminated.
Examples of thermally sensitive printing plates are described in U.S. Pat. No. 5,372,915 (Haley et al.). They include an imaging layer comprising a mixture of dissolvable polymers and an infrared radiation absorbing compound. While these plates can be imaged using lasers and digital information, they still require wet processing using alkaline developer solutions.
It has been recognized that a lithographic printing plate could be created by ablating an IR absorbing layer. For example, Canadian 1,050,805 (Eames) discloses a dry planographic printing plate comprising an ink receptive substrate, an overlying silicone rubber layer, and an interposed layer comprised of laser energy absorbing particles (such as carbon particles) in a self-oxidizing binder (such as nitrocellulose). Such plates were exposed to focused near IR radiation with a Nd++YAG laser. The absorbing layer converted the infrared energy to heat thus partially loosening, vaporizing or ablating the absorber layer and the overlying silicone rubber. Similar plates are described in Research Disclosure 19201, 1980 as having vacuum-evaporated metal layers to absorb laser radiation in order to facilitate the removal of a silicone rubber overcoated layer. These plates were developed by wetting with hexane and rubbing. Other publications describing ablatable printing plates include U.S. Pat. No. 5,385,092 (Lewis et al.), U.S. Pat. No. 5,339,737 (Lewis et al.), U.S. Pat. No. 5,353,705 (Lewis et al.), U.S. Reissue No. 35,512 (Nowak et al.), and U.S. Pat. No. 5,378,580 (Leenders).
While the noted printing plates used for digital, processless printing have a number of advantages over the more conventional photosensitive printing plates, there are a number of disadvantages with their use. The process of ablation creates debris and vaporized materials that must be collected. The laser power required for ablation can be considerably high, and the components of such printing plates may be expensive, difficult to coat, or unacceptable for resulting printing quality. Such plates generally require at least two coated layers on a support.
U.S. Pat. No. 3,650,743 (Hallman et al.) and U.S. Pat. No. 4,115,127 (Ikeda et) describe printing plates that do not require post-imaging processing. However, these imaging members are constructed of toxic materials such as arsenic that must be applied by vacuum deposition of mixed inorganic coating formulations.
Thermal or laser mass transfer is another method of preparing processless lithographic printing plates. Such methods are described for example in U.S. Pat. No. 5,460,918 (Ali et al.) wherein a hydrophobic image is transferred from a donor sheet to a microporous hydrophilic crosslinked silicated surface of the receiver sheet. U.S. Pat. No. 3,964,389 (Peterson) describes a process of laser transfer of an image from a donor material to a receiver material requiring a high temperature post-heating step.
Still another method of imaging that avoids chemical processing is the use of materials comprising microencapsulated hydrophobic materials as described for example in U.S. Pat. No. 5,569,573 (Takahashi et al.). Upon thermal imaging, the microcapsules rupture in an imagewise fashion to provide an ink-receptive image.
Thermally switchable polymers have been described for use as imaging materials in printing plates. By xe2x80x9cswitchablexe2x80x9d is meant that the polymer is rendered from hydrophobic to relatively more hydrophilic or, conversely from hydrophilic to relatively more hydrophobic, upon exposure to heat. U.S. Pat. No. 4,034,183 (Uhlig) describes the use of high powered lasers to convert hydrophilic surface layers to hydrophobic surfaces. A similar process is described for converting polyamic acids into polyimides through a transparency mask in U.S. Pat. No. 4,081,572 (Pacansky). The use of high-powered lasers is undesirable in the industry because of their high electrical power requirements and because of their need for cooling and frequent maintenance.
U.S. Pat. No. 4,634,659 (Esumi et al.) describes imagewise irradiating hydrophobic polymer coatings to render exposed regions more hydrophilic in nature. While this concept was one of the early applications of converting surface characteristics in printing plates, it has the disadvantages of requiring long UV light exposure times (up to 60 minutes), and the plate""s use is in a positive-working mode only.
U.S. Pat. No. 4,405,705 (Etoh et al.) and U.S. Pat. No. 4,548,893 (Lee et al.) describe amine-containing polymers for photosensitive materials used in non-thermal processes. Thermal processes using polyamic acids and vinyl polymers with pendant quaternary ammonium groups are described in U.S. Pat. No. 4,693,958 (Schwartz et al.). Such materials require aqueous processing after imaging. U.S. Pat. No. 5,512,418 (Ma) describes the use of polymers having cationic quaternary ammonium groups that are heat-sensitive. However, the materials described in this art require wet processing after imaging.
WO 92/09934 (Vogel et al.) describes photosensitive compositions containing a photoacid generator and a polymer with acid labile tetrahydropyranyl or activated ester groups. However, imaging of these compositions converts the imaged areas from hydrophobic to hydrophilic in nature.
EP-A 0 652 483 (Ellis et al.) describes direct-write lithographic printing plates imageable using IR lasers, and that do not require wet processing. These plates comprise an imaging layer that becomes more hydrophilic upon imagewise exposure to heat. This coating contains a polymer having pendant groups (such as t-alkyl carboxylates) that are capable of reacting under heat or acid to form more polar, hydrophilic groups. Imaging such compositions converts the imaged areas from hydrophobic to relatively more hydrophilic in nature, and thus requires imaging the background of the plate, which is generally a larger area. This can be a problem when imaging to the edge of the printing plate is desired. Moreover, the polymers described by Ellis et al. suffer from short shelf life and are difficult to manufacture.
The graphic arts industry is seeking an alternative means for providing processless, direct-write, negative-working lithographic imaging members that can be imaged without ablation, or the other problems noted above, to provide high sensitivity, high imaging speed, long shelf life, and long press life.
The problems noted above are overcome with a heat-sensitive composition comprising:
a) a thermally sensitive compound comprising a heat-activatable aromatic cyclic sulfonium zwitterion group represented by the following Structure I: 
wherein Ar is a substituted or unsubstituted aromatic group, X and Y are independently substituted or unsubstituted methylene groups, and n is 1 or 2, and
b) a photothermal conversion material.
This invention also provides a thermally sensitive imaging member comprising a support having disposed thereon an imaging layer comprising the thermally sensitive compound of Structure I described above, and a photothermal conversion material in the same or different layer.
This invention further provides a thermally sensitive imaging member comprising a support and having disposed thereon a heat-sensitive imaging layer that is prepared from the heat-sensitive composition described above.
Still further, this invention includes a method of imaging comprising the steps of:
A) providing an imaging member as described above, and
B) imagewise exposing the imaging member to thermal energy to provide exposed and unexposed areas in the imaging layer of the imaging member, whereby the exposed areas are rendered more hydrophobic than the unexposed areas.
A method of printing comprises the steps of carrying out steps A and B noted above, and additionally:
C) with or without wet processing, contacting the imagewise exposed imaging member with a lithographic printing ink, and imagewise transferring that printing ink from the imaging member to a receiving material.
The negative-working imaging members of this invention have a number of advantages and avoid the problems of previous printing plates. Specifically, the problems and concerns associated with ablation imaging (that is, imagewise removal of a surface layer) are avoided because imaging is accomplished in the imaging layer by xe2x80x9cswitchingxe2x80x9d (preferably, irreversibly) exposed relatively hydrophilic areas of the printing surface to a more hydrophobic (more ink-receptive) nature upon heating. Thus, the imaging layer stays intact during and after imaging (that is, no ablation occurs).
The thermally sensitive compounds having a heat-activatable group represented by Structure I used in the practice of this invention (both polymers and small molecule compounds) can be readily prepared or purchased from a number of commercial sources. Thus, the imaging members are simple to make and can be used with or without post-imaging wet processing. The resulting printing members formed from the imaging members of this invention are generally negative working in nature. Moreover, the imaging members are durable because upon heating, the heat-sensitive compounds described above not only provide hydrophobicity, but they react with each other, or crosslink, in the exposed areas.
Definitions:
xe2x80x9cPhotothermal conversion materialsxe2x80x9d are inorganic or organic compounds that absorb radiation from an appropriate energy source (such as a laser) and converts that radiation into heat. More details of such compounds are provided below.
As known in the lithographic printing art, materials that release or repel oil-based inks are referred to as having xe2x80x9coleophobicxe2x80x9d, xe2x80x9chydrophilicxe2x80x9d, or xe2x80x9cink-repellingxe2x80x9d character, and conversely, materials that accept oil-based inks are referred to an xe2x80x9coleophilicxe2x80x9d or xe2x80x9chydrophobic.xe2x80x9d
xe2x80x9cWet processingxe2x80x9d refers to washing off unexposed regions of the imaging layer after imaging using water or a fountain solution. It does not refer to contacting the imaging member with alkaline developers or other chemical processing solutions used in conventional lithographic developing methods.
The imaging members of this invention comprise a support and one or more layers thereon that include a dried thermally sensitive composition as described herein. The support can be any self-supporting material including polymeric films, glass, ceramics, cellulosic materials (including papers), metals or stiff papers, or a lamination of any of these materials. The thickness of the support can be varied and should be sufficient to sustain the wear from printing and thin enough to wrap around a printing form. A preferred embodiment uses a polyester support prepared from, for example, polyethylene terephthalate or polyethylene naphthalate, and having a thickness of from about 100 to about 310 xcexcm. Another preferred embodiment uses aluminum sheets having a thickness of from about 100 to about 600 lm. The support should resist dimensional change under conditions of use. The aluminum and polyester supports are most preferred for the imaging members of this invention.
The support may also be a cylindrical support that includes printing cylinders on press as well as printing sleeves that are fitted over printing cylinders. The use of such supports to provide cylindrical imaging members is described in U.S. Pat. No. 5,713,287 (Gelbart). The thermally sensitive composition of this invention can be coated or sprayed directly onto the cylindrical surface that is an integral part of the printing press.
The support may be coated with one or more xe2x80x9csubbingxe2x80x9d layers to improve adhesion of the final assemblage. Examples of subbing layer materials include, but are not limited to, gelatin and other naturally occurring and synthetic hydrophilic colloids and vinyl polymers (such as vinylidene chloride copolymers) that are known for such purposes in the photographic industry, vinylphosphonic acid polymers, sol gel materials such as those prepared from alkoxysilanes (including glycidoxypropyltriethoxysilane and aminopropyltriethoxysilane), epoxy functional polymers, and various ceramics. The support can also have thereon conventional reflecting layers such as layers of evaporated metals, or an IR radiation reflection layer.
The backside of the support may be coated with antistatic agents and/or slipping layers or matte layers to improve handling and xe2x80x9cfeelxe2x80x9d of the imaging member.
The imaging members, however, preferably have only one layer on the support, that is a heat-sensitive surface layer that is required for imaging. This layer is prepared from a heat-sensitive composition of this invention and includes one or more thermally sensitive compounds and one or more photothermal conversion materials (both described below) as the only essential components for imaging. Because of the particular thermally sensitive compounds used in the imaging layer, the exposed (imaged) areas of the layer are rendered more hydrophobic in nature. The unexposed areas remain relatively hydrophilic in nature and can be washed off using water or a fountain solution if desired.
In an alternative embodiment, the imaging member comprises one or more thermally sensitive compounds as described herein in a surface imaging layer, and one or more photothermal conversion materials in a separate layer directly underneath, or in thermal contact with, the imaging layer. The photothermal conversion materials can diffuse into the imaging layer prior to or during imaging.
The thermally sensitive compounds useful in the imaging members of this invention can be one or more polymers or one or more small molecular compounds (non-polymeric), or the blend of both types of compounds. Useful heat-sensitive polymers can be vinyl homopolymers or copolymers prepared from one or more ethylenically unsaturated polymerizable monomers that are reacted together using known polymerization techniques. Alternatively, they can be addition homopolymers or copolymers prepared from one or more heterocyclic monomers that are reacted together using known polymerization techniques (such as polyethers or polythioethers). Additionally, they can be condensation type polymers (such as polyesters, polythioester, polyimides, polyamides, polyurethanes, polyketones, polycarbonates, polyanhydrides, polysulfones, polysulfines, polyureas, or phenol-formaldehyde resin) prepared using known polymerization procedures and conditions.
Whether the compounds are small molecular compounds or polymers, they comprise one or more heat-activatable aromatic cyclic zwitterion groups as represented by the following Structure I: 
wherein Ar is a substituted or unsubstituted aromatic carbocyclic or heterocyclic group that generally has from 6 to 18 carbon atoms in the aromatic ring system (such as phenyl, naphthyl, anthryl, phenanthryl, pyrenyl, crysenyl, triphenyl, benzanthryl, and biphenyl). Preferably, Ar is a substituted or unsubstituted phenyl or naphthyl group. Depending upon the type of thermally sensitive compound being used, Ar can represent either a monovalent or divalent aromatic group (for example, arylxe2x80x9d or xe2x80x9carylenexe2x80x9d group). The various substituents on Ar can be any of those defined below for the xe2x80x9cRxe2x80x9d group for Structure II.
Structure I also represents dimer compounds in which two of the illustrated moieties are directly linked through the Ar groups, or by means of a linking group like Z defined below.
If the thermally sensitive compound is a thermally sensitive polymeric material, it can be represented by the following Structure II: 
wherein A represents a polymeric backbone from any of the type of polymers described above. Preferably, A represents a vinyl polymer binder.
In Structure II, Arxe2x80x2 represents a divalent aromatic group that is defined similar to Ar. R represents the same or different substituent on said aromatic group. Representative useful R groups are defined below. Moreover, m is 0 or an integer representing available open ring positions on Arxe2x80x2. Preferably, m is 0 or 1 to 3, and more preferably, m is 0, 1 or 2.
The heat-activatable aromatic cyclic sulfonium zwitterion group can be directed attached to the polymeric backbone (that is, q is 0), or it can be attached by means of the divalent linking group xe2x80x9cZxe2x80x9d (that is, q is 1). Preferably, q is 1.
Z is xe2x80x94(Zxe2x80x2)pZxe2x80x3txe2x80x94 wherein Zxe2x80x2 is an oxy (xe2x80x94Oxe2x80x94), thio (xe2x80x94Sxe2x80x94), carbonyloxy (xe2x80x94COOxe2x80x94), oxycarbonyl (xe2x80x94OCOxe2x80x94), carbonyl (xe2x80x94COxe2x80x94), carbonyloxycarbonyl (xe2x80x94COOCOxe2x80x94), xe2x80x94SOxe2x80x94, xe2x80x94SO2, xe2x80x94NHCONHxe2x80x94, or carbonamido (xe2x80x94CONHxe2x80x94) group. Preferably, Zxe2x80x2 is a carbonyloxy group. The hydrogen atoms on the carbonamido and xe2x80x94NHCONHxe2x80x94 groups can be replaced with any of the groups described above for R.
Zxe2x80x3 is a substituted or unsubstituted alkylene, substituted or unsubstituted arylene, substituted or unsubstituted arylenealkylene, or substituted or unsubstituted alkylenearylene group, each group having up to 20 carbon atoms in the chain linking the polymer backbone to the heat-activatable group. Thus, representative Zxe2x80x3 groups include, but are not limited to, substituted or unsubstituted methylene, ethylene, n-butylene, isopropylene, phenylene, naphthylene, anthracylene, xylylene, p-methylenephenylene, p-phenylenemethylene, phenylenemethylenephenylene, biphenylene, phenyleneisopropylenephenylene, 1,4-dimethylenephenylene, and terphenylene. Preferably, Zxe2x80x3 is a substituted or unsubstituted alkylene or arylene group, and more preferably, it is a substituted or unsubstituted alkylene group having 1 to 6 carbon atoms (including linear and branched groups), a substituted phenylene or naphthylene group, or a substituted or unsubstituted phenylenealkylene group having 7 or 8 atoms in the linking chain.
In addition, p and t are independently 0 or 1, and preferably, each is 1.
More preferred Z groups are xe2x80x94COOxe2x80x94 or xe2x80x94COOxe2x80x94Zxe2x80x3 groups wherein Zxe2x80x3 is an alkylene group having 1 or 2 carbon atoms, a phenylene group, or a naphthylene group. Most preferably, Z is xe2x80x94COOxe2x80x94.
Ar and Arxe2x80x2 can be substituted with one or more monovalent substituents, the number and type of substituents are limited only by the available valences on the aromatic ring and they do not adversely affect the desired imaging properties of the thermally sensitive compounds. For example, such substituents (shown as xe2x80x9cRxe2x80x9d in Structure II) can be a halo (such as fluoro, chloro, or bromo) group, an alkyl group generally having 1 to 16 carbon atoms (including linear and branched monovalent alkyl groups that can be further substituted with any substituents described herein), an aryl group generally having 1 to 18 carbon atoms in the ring system (including those groups described for Ar), an alkoxy group generally having 1 to 16 carbon atoms (including linear and branched monovalent alkyl groups that can be substituted with any of the substituents described herein), a substituted or unsubstituted alkyl or aryl ester group (such as carbonyloxymethyl, carbonyloxyphenyl, oxycarbonylethyl, and oxycarbonylphenyl), a primary, secondary or tertiary amine, a thioether group, or a thioester group.
Preferably, Ar and Arxe2x80x2 are substituted with 1 to 3 chloro, bromo, or substituted or unsubstituted alkyl groups having 1 to 4 carbon atoms (both linear and branched groups).
In both Structures I and II, X and Y are independently substituted or unsubstituted methylene groups (such as xe2x80x94CH2xe2x80x94 or xe2x80x94CHRxe2x80x94), and n is 1 or 2, wherein R is as defined above. Preferably, each of X and Y is xe2x80x94CH2xe2x80x94, and n is 1.
Representative small molecule thermally sensitive compounds useful in the present invention from Structure I can be illustrated by the following types of compounds of Structures I-a through I-k wherein R, Z, q, and m are defined above: 
Mixtures of such compounds can be used if desired in the imaging members of this invention.
The following non-polymeric, small molecule Compounds 1-5 useful in the present invention were specifically prepared in the Synthetic Examples described below: 
The heat-sensitive polymers useful in the practice of this invention include polymer recurring units that comprise the heat-activatable aromatic cyclic sulfonium zwitterion groups as illustrated in Structure II above. The polymers can include more than one type of recurring unit that comprise such groups. In addition, they can include polymer recurring units that do not comprise such groups. Generally, at least 10 mol %, preferably from about 15 to 100 mol %, and more preferably from about 20 to 100 mol %, of all recurring units in the polymers comprise the noted heat-activatable groups. The recurring units can be derived from polymerizable monomers that contain the heat-activatable groups, or they can be derived from monomers that, after polymerization, can be modified to provide the heat-activatable groups. Mixtures of such polymers can be used in the imaging members of this invention if desired.
Preferred heat-sensitive polymers are vinyl polymers derived from one or more ethylenically unsaturated polymerizable monomers and can be represented by the following Structure III: 
wherein Xxe2x80x2 represents recurring units to which the heat-activatable aromatic cyclic sulfonium zwitterion groups of Structure I (represented by xe2x80x9cSULFOxe2x80x9d in Structure III) are attached, and Yxe2x80x2 represents recurring units derived from any additional ethylenically unsaturated polymerizable monomers. The various repeating units are present in suitable amounts, as represented by x being from about 10 to 100 mol %, and y being from 0 to about 90 mol %. Preferably, x is from about 20 to 100 mol %, and y is from 0 to about 80 mol %.
While the heat-activatable groups in the polymers are crosslinked in imaged areas of the imaging member, additional crosslinking can be provided in a number of ways if desired as long as this crosslinking does not interfere with the transformation of the aromatic cyclic sulfonium zwitterion groups during imaging. There are numerous monomers and methods for crosslinking that are familiar to one skilled in the art. Some representative crosslinking strategies include, but are not necessarily limited to:
a) reacting an amine or carboxylic acid or other Lewis basic units with diepoxide crosslinkers,
b) reacting epoxide units within the polymer with difunctional amines, carboxylic acids, or other difunctional Lewis basic unit,
c) irradiative or radical-initiated crosslinking of double bond-containing units such as acrylates, methacrylates, cinnamates, or vinyl groups,
d) reacting a multivalent metal salts with ligating groups within the polymer (the reaction of zinc salts with carboxylic acid-containing polymers is an example),
e) using crosslinkable monomers that react via the Knoevenagel condensation reaction, such as (2-acetoacetoxy)ethyl acrylate and methacrylate,
f) reacting an amine, thiol, or carboxylic acid groups with a divinyl compound (such as bis (vinylsulfonyl) methane) via a Michael addition reaction,
g) reacting a carboxylic acid units with crosslinkers having multiple aziridine units,
h) reacting a crosslinkers having multiple isocyanate units with amines, thiols, or alcohols within the polymer,
i) mechanisms involving the formation of interchain sol-gel linkages [such as the use of the 3-(trimethoxysilyl) propylmethacrylate monomer],
j) oxidative crosslinking using an added radical initiator (such as a peroxide or hydroperoxide),
k) autooxidative crosslinking, such as employed by alkyd resins,
l) sulfur vulcanization, and
m) processes involving ionizing radiation.
Polymerizable monomers having crosslinkable groups or active crosslinkable sites (or groups that can serve as attachment points for crosslinking additives, such as epoxides) can be copolymerized with the other monomers noted above. Such polymerizable monomers include, but are not limited to, 3-(trimethoxysilyl)propyl acrylate or methacrylate, cinnamoyl acrylate or methacrylate, N-methoxymethyl methacrylamide, N-aminopropylacrylamide hydrochloride, acrylic or methacrylic acid and hydroxyethyl methacrylate.
Other additional polymerizable monomers that can provide the recurring units represented by xe2x80x9cYxe2x80x9d in the Structure III above include any useful hydrophilic or oleophilic ethylenically unsaturated polymerizable monomer that may provide desired physical or printing properties to the hydrophilic imaging layer. Such monomers include, but are not limited to, acrylates and methacrylates (such as ethyl acrylate, ethyl methacrylate, n-butyl acrylate, methyl methacrylate, t-butyl methacrylate, and n-butyl methacrylate), acrylonitrile and methacrylonitrile, styrene and styrene derivatives, acrylamides and methacrylamides, vinyl ethers, vinyl pyridines, vinyl pyrrolidones, vinyl acetate, vinyl halides (such as vinyl chloride, vinylidene chloride, and vinyl bromide), and dienes (such as ethylene, propylene, 1,3-butadiene, and isobutylene). Acrylates, acrylamides and styrene (and its derivatives) are preferred.
Preferred general polymeric thermally sensitive compounds useful in the present invention include, but are not limited to, those represented by the following Structures II-a to II-f: 
Mixtures of these polymers can be used in the imaging members of this invention if desired.
The following specific polymeric thermally sensitive compounds 6-8 were prepared in the Synthetic Examples below: 
A mixture of thermally sensitive compounds can also be used in the imaging layer. Such mixtures can include one or more non-polymeric, small molecule thermally sensitive compounds, one or more thermally sensitive polymers, or one or more small molecule compounds with one or more polymeric compounds.
The thermally sensitive compounds described herein having the aromatic cyclic sulfonium zwitterion groups are believed to polymerize and switch from relatively hydrophilic to relatively hydrophobic upon exposure to thermal energy that provides or generates heating. Hence, the imaging members of this invention are negative working imaging members.
Thermally sensitive compounds comprising five-membered cyclic sulfonium groups can be prepared by the reaction of a phenolic compound with tetrahydrothiophene 1-oxide and hydrogen chloride or with tetrahydrothiophene and chloride. The resulting hydrochloride salts are converted to the zwitterions using an ion-exchange resin or by neutralization using sodium methoxide. The compounds comprising six-membered cyclic sulfonium groups can be synthesized from the reaction of 1,5-dibromopentanes with methylthiophenolic compounds in refluxing chlorobenzene. The general synthetic procedures are described in U.S. Pat. No. 3,636,052 (Hatch), incorporated herein by reference. Polymers containing the heat-activatable aromatic cyclic sulfonium zwitterions can be prepared either from functional monomers or from preformed polymers as described above.
Representative synthetic methods for thermally sensitive polymers and small molecule compounds containing active aromatic cyclic sulfonium zwitterions useful in the practice of this invention are illustrated below just before the Examples.
The imaging layer of the imaging member can also include minor amounts (less than 20 weight %, based on total dry weight of the layer) of additional binder or polymeric materials that will not adversely affect its imaging or printing characteristics. However, the imaging layer comprises no additional materials that are needed for imaging, especially those materials (such as novolak or resole resins) commonly used for wet processing with alkaline developer solutions.
In the thermally sensitive composition (including solvent) used to provide the heat-sensitive layer, the amount of thermally sensitive compound is generally present in an amount of at least 1% solids, and preferably at least 2% solids. A practical upper limit of the amount of charged polymer in the composition is 90% solids.
The amount of thermally sensitive compound(s) present in the dried imaging layer is generally at least 0.1 g/m2, and preferably from about 0.1 to about 10 g/m2 (dry weight). The dried imaging layer generally has an average dry thickness of from about 0.1 to about 10 m. Greater amounts can be used if desired.
The imaging and any other layers in the imaging member can also include one or more conventional surfactants for coatability or other properties, dyes or colorants to allow visualization of the written image, or any other addenda commonly used in the lithographic art, as long as the concentrations are low enough so they are inert with respect to imaging or printing properties.
It is essential that the imaging member include one or more photothermal conversion materials. Preferably, they absorb radiation in the infrared and near-infrared regions of the electromagnetic spectrum. The photothermal conversion materials useful in this invention include infrared radiation (IR) dyes, carbon black, polymer grafted carbon, pigments, evaporated pigments, semiconductor materials, alloys, metals, metal oxides, metal sulfides or combinations thereof, or a dichroic stack of materials that absorb radiation by virtue of their refractive index and thickness. Borides, carbides, nitrides, carbonitrides, bronze-structured oxides and oxides structurally related to the bronze family but lacking the WO29 component, are also useful. Useful absorbing dyes for near infrared diode laser beams are described, for example, in U.S. Pat. No. 4,973,572 (DeBoer). Particular dyes of interest are xe2x80x9cbroad bandxe2x80x9d dyes, that is those that absorb over a wide band of the spectrum. Mixtures of one or more types of these compounds can be used if desired. Carbon blacks and IR dyes are preferred photothermal conversion materials.
Still other useful photothermal conversion materials include multisulfonated IR dyes as described U.S. Pat. No. 6,159,657 (Fleming et al.), the disclosure of which is incorporated herein by reference.
Useful IR-dyes are sensitive to radiation in the near-infrared and infrared regions of the electromagnetic spectrum. Thus, they are generally sensitive to radiation at or above 700 nm (preferably from about 800 to about 900 nm, and more preferably from about 800 to about 850 nm).
Examples of useful IR dyes of several classes include, but are not limited to, bis(dichlorobenzene-1,2-thiol)nickel(2:1)tetrabutyl ammonium chloride, tetrachlorophthalocyanine aluminum chloride, and the following compounds: 
IR Dye 2 is the same as IR Dye 1 but with C3F7CO2xe2x88x92 as the anion. 
IR Dyes 1-7 can be prepared using known procedures or obtained from several commercial sources (for example, Esprit, Sarasota, Fla.). IR dyes 8-14 can be prepared using known procedures, as described for example in U.S. Pat. No. 4,871,656 (Parton et al.) and reference noted therein (for example, U.S. Pat. No. 2,895,955, U.S. Pat. No. 3,148,187 and U.S. Pat. No. 3,423,207), all incorporated by reference.
As noted above, the one or more photothermal conversion materials can be included in a separate layer that is in thermal contact with the heat-sensitive imaging layer. Thus, during imaging, the action of the additional photothermal conversion material can be transferred to the heat-sensitive imaging layer.
Wherever the photothermal conversion materials are located, they are generally present in an amount sufficient to provide an optical density of at least 0.1, and preferably of at least 1.0. The particular amount required for a given material and formulation could be readily determined by a skilled worker in the art using routine experimentation. In the heat-sensitive formulation used to provide heat-sensitive imaging layers, the photothermal conversion material(s) is generally present in an amount of from about 5 to about 35% of the total solids (prior to drying).
The thermally sensitive composition of this invention can be applied to a support using any suitable equipment and procedure, such as spin coating, knife coating, gravure coating, dip coating or extrusion hopper coating. In addition, the composition can be sprayed onto a support, including an on-press cylindrical support (such as an on-press cylinder or sleeve), using any suitable spraying means for example as described in U.S. Pat. No. 5,713,287 (noted above) to provide an imaging member. Exposure can occur as in Step B described above.
The thermally sensitive compositions of this invention are generally formulated in and coated from water or water-miscible organic solvents including, but not limited to, water-miscible alcohols (for example, methanol, ethanol, isopropanol, 1-methoxy-2-propanol and n-propanol), methyl ethyl ketone, tetrahydrofuran, acetonitrile and acetone. Water, methanol, ethanol and 1-methoxy-2-propanol are preferred. Mixtures (such as a mixture of water and methanol) of these solvents can also be used if desired. By xe2x80x9cwater-misciblexe2x80x9d is meant that the organic solvent is miscible in water at all proportions at room temperature.
The imaging members of this invention can be of any useful form including, but not limited to, printing plates, printing cylinders, printing sleeves and printing tapes (including flexible printing webs), all of any suitable size or dimensions. Preferably, the imaging members are lithographic printing plates having an aluminum support or on-press cylinders.
During use, the imaging member of this invention is exposed to a suitable source of energy that generates or provides heat, such as a focused laser beam or a thermoresistive head (or xe2x80x9cthermal headxe2x80x9d), in the foreground areas where ink is desired in the printed image, typically from digital information supplied to the imaging device. A laser used to expose the imaging member of this invention is preferably a diode laser, because of the reliability and low maintenance of diode laser systems, but other lasers such as gas or solid state lasers may also be used. The combination of power, intensity and exposure time for laser imaging would be readily apparent to one skilled in the art. Specifications for lasers that emit in the near-IR region, and suitable imaging configurations and devices are described in U.S. Pat. No. 5,339,737 (Lewis et al.), incorporated herein by reference. The imaging member is typically sensitized so as to maximize responsiveness at the emitting wavelength of the laser.
The imaging apparatus can operate on its own, functioning solely as a platemaker, or it can be incorporated directly into a lithographic printing press. In the latter case, printing may commence immediately after imaging, thereby reducing press set-up time considerably. The imaging apparatus can be configured as a flatbed recorder or as a drum recorder, with the imaging member mounted to the interior or exterior cylindrical surface of the drum.
In the drum configuration, the requisite relative motion between an imaging device (such as laser beam) and the imaging member can be achieved by rotating the drum (and the imaging member mounted thereon) about its axis, and moving the imaging device parallel to the rotation axis, thereby scanning the imaging member circumferentially so the image xe2x80x9cgrowsxe2x80x9d in the axial direction. Alternatively, the beam can be moved parallel to the drum axis and, after each pass across the imaging member, increment angularly so that the image xe2x80x9cgrowsxe2x80x9d circumferentially. In both cases, after a complete scan by the laser beam, an image corresponding to the original document or picture can be applied to the surface of the imaging member.
In the flatbed configuration, a laser beam is drawn across either axis of the imaging member, and is indexed along the other axis after each pass. Obviously, the requisite relative motion can be produced by moving the imaging member rather than the laser beam.
While laser imaging is preferred in the practice of this invention, imaging can be provided by any other means that provides or generates thermal energy in an imagewise fashion. For example, imaging can be accomplished using a thermoresistive head (thermal printing head) in what is known as xe2x80x9cthermal printingxe2x80x9d, described for example in U.S. Pat. No. 5,488,025 (Martin et al.). Such thermal printing heads are commercially available (for example, as Fujisu Thermal Head FTP-040 MCS001 and TDK Thermal Head F415 HH7-1089).
Imaging on printing press cylinders can be accomplished using any suitable means, for example, as taught in U.S. Pat. No. 5,713,287 (noted above), that is incorporated herein by reference.
After imaging, the imaging member can be used for printing without conventional wet processing. Unexposed areas in the imaging surface can be washed away if desired using a conventional fountain solution. Applied ink can be imagewise transferred to a suitable receiving material (such as cloth, paper, metal, glass or plastic) to provide one or more desired impressions. If desired, an intermediate blanket roller can be used to transfer the ink from the imaging member to the receiving material. The imaging members can be cleaned between impressions, if desired, using conventional cleaning means.
The following synthetic examples are presented to show how some of the preferred thermally sensitive compounds can be prepared.